As for bearings used for an automobile, an expensive ball bearing is used for high load applications, such as a bearing for an ABS system of an automobile, while an inexpensive Fe—Cu-based sintered bearing is used for a motor system of an automobile wiper or the like. Due to reduction in size of the motor system, however, the bearing for the motor system is also progressively reduced in size to thereby increase a load applied to the bearing parts. Therefore, ever more excellent performances in wear resistance and seizure resistance are required for the bearing parts.
Recently, to meet strong demands for cost reduction from the market, employing an inexpensive sintered bearing instead of an expensive ball bearing is under consideration even for high load applications such as the foregoing ABS system of the automobile. When using the conventional Cu-based sintered sliding member, however, a load applied thereto is too heavy for it and hence the load exceeds its allowable load range, thus making it impossible to use the member. Further, Fe—Cu based sintered sliding members whose hardness and strength are higher than those of the Cu-based sintered sliding members contain Fe, while a shaft borne by the Cu-based sintered sliding member is also made of an Fe-based metal, and thus, abnormal friction and seizure are likely to occur due to the same-metal phenomenon, even though the probability of the occurrence thereof is low, thus leading to the problem that the reliability thereof as a sliding member is insufficient. Thus, a Cu-based sintered sliding member which is less expensive than the expensive ball bearings and is capable of being used under higher load conditions than in the past has been sought after.
As a conventional art with respect to the Cu-based sintered sliding member usable under high load conditions, there is disclosed a Cu-based sintered sliding member (in e.g., Japanese unexamined patent application publication No. H5-195117) having excellent wear resistance and seizure resistance under high-temperature, high-load and poor-lubrication conditions when it is used for a valve guide or the like of an internal-combustion engine.
The Cu-based sintered alloy according to the conventional art is a Cu—Ni—Sn based alloy whose composition gives rise to a spinodal decomposition through an aging treatment. By undergoing the spinodal decomposition, the Cu—Ni—Sn based alloy is allowed to be formed with a microstructure to thereby strengthen its metallic substrate. Further, through the addition of Ni-based hard particles having excellent adhesiveness to the metallic substrate along with MoS2 as a solid lubricant, wear resistance and seizure resistance are imparted thereto under high-temperature, high-load and poor-lubrication conditions.
The Ni-based hard particles used in the conventional art, however, are not only expensive but a vacuum sintering process is required therefor as Cr is contained in the Ni-based hard particles, thus leading to high manufacturing cost, resulting in an insufficient cost advantage.